ltpstress.sh: added cmdline to set the block device

I think it's not needed to use set_block_device(),
create_block_device() from runltp, because their logic
is in tst_acquire_device()/tst_release_device() now.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>